Very nice looking for your first attempt.
I use plastic welding rods in the corners for bathtub sheets. After your first row of stitching for the corners, I place a 2nd row just far enough apart of the plastic rod and then sew it closed after inserting. You can pick up a large package of them for like 5 bucks at a hardware store and they are pretty easy to cut with a pair of scissors.
A shock cord loop at the tie outs would help to maintain tension on the sides as the silnylon stretches.
I remember my first stuff sack I made. You couldn't find a straight stitch anywhere. I actually still have it. I keep all of my sewing feet in it. In fact, I just looked at the stitching this morning and shook my head.